Why I'm doing:

This is the further improvement on this PR: #50686

We find a bad case that

• if there are many scan nodes
• but most of them does not create any chunk source
• the scan node which has chunk source uses a very few io tasks

And this case can be reproduced by following SQL

• only select lo_orderkey from ICE has data
• the MYSQL does not have any data.

set cbo_cte_reuse = false;
with ICE as (
 select lo_orderkey from iceberg.zz_iceberg_ssb_sf100_iceberg_parquet_lz4.lineorder_flat
),
MYSQL(id_int) as (
  select id_int from default_catalog.zya.ext_mysql where id_varchar = 'USA'
),
RESULT(x) as (
select lo_orderkey from ICE
UNION ALL (select lo_orderkey from ICE inner join [broadcast] MYSQL on ICE.lo_orderkey = MYSQL.id_int)
UNION ALL (select lo_orderkey from ICE inner join [broadcast] MYSQL on ICE.lo_orderkey + 1= MYSQL.id_int)
UNION ALL (select lo_orderkey from ICE inner join [broadcast] MYSQL on ICE.lo_orderkey + 2= MYSQL.id_int)
UNION ALL (select lo_orderkey from ICE inner join [broadcast] MYSQL on ICE.lo_orderkey + 3= MYSQL.id_int)
UNION ALL (select lo_orderkey from ICE inner join [broadcast] MYSQL on ICE.lo_orderkey + 4= MYSQL.id_int)
)
select count(distinct x) from RESULT;

So the execution profile looks like this

And if you look at PeakIOTasks of ICE table, it's very low probably like 3-4, which is bad.

The root cause is, since we have this PR: #50686

• we preallocate $$ScanNodeNumber * 256MB$$ as total memory pool.
• and each scan operator will adjust their chunk source usage from 256MB when creating chunk source.
• and at last, each scan operator will release their chunk source usage down to 0 when doing close
• and during this adjustment, those scan operators are sharing memory limit according to chunk source usage.

However, there is a corner case that: if the scan operator does not create any chunk source, it has no chance to adjust it's chunk usage back to 0. And it affects other scan operator's available mem limit, which leads to low io tasks.

What I'm doing:

This PR is to:

• adjust chunk source usage when do_preapre and do_close. so it will do adjustment even there is no chunk source created.
• at FE side, we change scan_node_number to more proper name connector_scan_node_number. So there won't be problems if there are no-connector scan node and connector scan nodes in a single query.

Fixes #50686

What type of PR is this:

• BugFix
• Feature
• Enhancement
• Refactor
• UT
• Doc
• Tool

Does this PR entail a change in behavior?

• Yes, this PR will result in a change in behavior.
• No, this PR will not result in a change in behavior.

If yes, please specify the type of change:

• Interface/UI changes: syntax, type conversion, expression evaluation, display information
• Parameter changes: default values, similar parameters but with different default values
• Policy changes: use new policy to replace old one, functionality automatically enabled
• Feature removed
• Miscellaneous: upgrade & downgrade compatibility, etc.

Checklist:

• I have added test cases for my bug fix or my new feature
• This pr needs user documentation (for new or modified features or behaviors)
  • I have added documentation for my new feature or new function
• This is a backport pr

Bugfix cherry-pick branch check:

• I have checked the version labels which the pr will be auto-backported to the target branch
  • 3.4
  • 3.3
  • 3.2
  • 3.1
  • 3.0